The present invention relates to the general field of nozzles fitted to turbomachines. The invention relates more particularly to a separate stream nozzle having at least one of its cowls provided with patterns for reducing the jet noise generated on leaving the nozzle.
Separate stream nozzles for turbomachines are generally made up of a primary cowl, a secondary cowl disposed concentrically around the primary cowl so as to define a first annular channel for passing an outer stream (or cold stream), and a central body disposed concentrically inside the primary cowl so as to define a second annular channel for passing an inner stream (or hot stream).
One of the known solutions for reducing jet noise at the outlet from such a nozzle is to encourage mixing between the hot and cold streams coming from the turbomachine. The difficulty of the problem lies in controlling the characteristics of the mixing to be obtained between the hot and cold streams, given that one consequence of mixing too suddenly is an undesirable increase in levels of turbulence in the near field of the discharge. Such an increase has a negative influence on the potential for reducing noise in mixing zones that are further away. Thus, mixing between the streams must be as effective as possible, while satisfying constraints and criteria concerning both aerodynamic and acoustic efficiency.
To this end, it is well known to provide one of the cowls of the nozzle with a plurality of repeated patterns distributed around the entire circumference of the trailing edge of the cowl. By putting such patterns into place at the trailing edge of the nozzle cowl, the streams are caused to mix together by creating contrarotating longitudinal vortices.
For example, European patent application EP 0 913 567 provides for fitting the trailing edge of the primary cowl of the nozzle with a plurality of repeated patterns of triangular shape (referred to as chevrons) that encourage mixing between the hot and cold streams. Similarly, publication GB 2 355 766 proposes providing the trailing edges of the primary and secondary cowls of the nozzle with a plurality of repeated patterns of trapezoidal shape (refereed to as crenellations).
Although encouraging mixing between these streams, the patterns mentioned above nevertheless present drawbacks. A pattern of symmetrical shape (such as triangles or crenellations) at the trailing edge of at least one of the cowls of the nozzle causes each pattern to generate two contrarotating longitudinal vortices of equivalent intensity that are relatively close to each other. Over the entire circumference of the nozzle cowl, that is equivalent to a plurality of pairs of vortices that compensate mutually. That results in mixing between the streams that is not very effective, particularly in the zones that are furthest away from the ejection.